Iron golf clubs are well known which are made of a composite material composed of a low specific gravity metal such as titanium or titanium alloy, and a high specific gravity metal such as plain steel or stainless steel.
A golf club head made of a composite material, in which a low specific gravity metal and a high specific gravity metal are bonded to each other, is characterized in that the entire mass of the club head can be reduced. Therefore, it is possible to provide a design thereof with the following advantages.
(1) While the weight of the golf club head is provided with the same value, the size of the club head can be increased.
(2) It is possible to locate the center of gravity at a position which is low with respect to the sole of the golf club head.
(3) It is possible to increase the depth of the center of gravity from the face outer of the golf club head.
(4) It is possible to extend a range in which the sweet spot is located.
However, usually, it is difficult to bond a low specific gravity metal and a high specific gravity metal so as to bond these two types of metals by melting, fusing such as welding. Therefore, the following methods can be adopted. One is a method in which a recess is formed in a portion of the club head and the pieces of metal, which are different from each other, are made to adhere to each other by a bonding agent in the recess (Japanese Unexamined Patent Publication (Kokai) No. 6-165843). The other is a method in which the pieces of metals are press-fitted into the recess so that the pieces of metal can be mechanically engaged with each other (Japanese Unexamined Patent Publication (Kokai) Nos. 6-182006 and 6-246021). However, the following problems may be encountered in the above methods.
First, when pieces of metal, which are different from each other, are joined by bonding agent, the following problems may be encountered. (1) Since the joining strength is low, there is a possibility that the joining surfaces are separated when an impact force is repeatedly applied to the club head face in the case of hitting golf balls. (2) When the golf club is heated after the completion of adhesion, the joined surfaces are separated by the action of heat. Therefore, it is impossible to conduct thermal working on the club head after the completion of adhesion. (3) A bonding agent applied to the club head is deteriorated by the action of heat generated in the working of polishing the club head. Accordingly, it is necessary to suppress the generation of heat in the prudent working of polishing. As a result, it is difficult to conduct polishing in a short period of time, and the working cost is increased. (4) In order to solve the above problems, there is a method for using a pin or the like to fix the pieces of metals. However, this method requires complicated work, and the working cost is increased.
Secondly, when the mechanical engaging means for joining the pieces of metals such as press-fitting is adopted, the engaging portion is loosened in a high temperature environment in summer due to a difference in the physical properties of the material of the club head body and the material of the club face, for example, due to a difference in the characteristics of thermal expansion. Accordingly, there is a possibility that the pieces of metal are disconnected from each other. For the above reasons, the usable combination of metals is restricted.
On the other hand, there is proposed a method in which a composite material is made by means of explosive-welding so that different metals can be metallurgically bonded to each other and thus obtained composite material is subjected to forging so as to form it into a predetermined shape (Japanese Examined Patent Publication (Kokoku) No. 6-26634). When composite material is made by means of explosive-welding and the shape of a club head is formed by forging as described above, it is possible to solve the above problems caused when composite material is made by means of joining by bonding agent and engaging, however, the following different problems may be encountered.
Concerning the method of forging, there are provided a hot forging method and a cold forging method. (1) In the hot forging method, carbon and nitrogen are diffused on the bonding interface in the process of heating. Therefore, carbide and nitride are generated on the bonding interface, and the bonding strength is lowered. (2) In the cold forging method, due to a difference in the plastic deformability between both metals to be bonded, there exists a work strain on the bonding interface in the process of forging. When this work strain is intense, interface separation may be caused by the working and the bonding strength may become very unstable. For example, when pure titanium is used as a low specific gravity metal and stainless steel is used as a high specific gravity metal, the bonding strength obtained by this method is locally deteriorated and the bonding surface is separated by the impact force repeatedly applied when golf balls are hit by this club head. (3) Concerning the method of forging conducted on metals different from each other, it is possible to form the metals into a predetermined shape either by the hot forging method or the cold forging method. However, due to a difference in the plastic deformability between both metals to be bonded, it is difficult to conduct forming while the distribution of weight is kept constant. Therefore, it is necessary to prudently adjust the balance of weight in the process of polishing, which takes time and the working cost is increased.
Further, when different metals are bonded to each other in such a manner that a low specific gravity metal is arranged on the front face side of the club head including the club face and that a high specific gravity metal is arranged on the back face side of the club head, it is possible to provide a club head, the depth of the center of gravity of which is increased and the sweet spot range of which is extended. However, when the depth of the center of gravity is increased as required, there is a problem that the thickness of the club head is increased, and the weight of the club head becomes excessive.